In my prior U.S. Pat. Nos. 3,717,827 and 3,815,137 issued on Feb. 20, 1973 and June 4, 1974 respectively, as well as in prior U.S. Pat. No. 3,124,768 issued Mar. 10, 1964, interference problems in the field of radio communication are discussed. Briefly, these problems involve the simultaneous utilization of one antenna or transmission line with two or more transmitting and receiving pieces of equipment operating at carrier signals of different frequencies such as are found in multicouplers in general and in diplexers and duplexers specifically. My prior co-pending patent application Ser. No. 826,412 filed Aug. 22, 1977 is also concerned with and is directed to the design of filters and multicouplers assembled therefrom.
In order to properly isolate various pieces of equipment from one another, a number of filter networks are commonly utilized as is taught in the multicoupler of U.S. Pat. No. 3,124,768. Each such network includes a first cavity resonator and a quarter wavelength transmission line tuned to pass only the frequency of the signaling device connected to the network, and a second cavity resonator and a second quarter wavelength transmission line tuned to block only the frequency of the signaling device and to pass the frequencies of the other signaling devices. Each of the second cavity resonators and second transmission lines are connected in series and in turn are connected to the common antenna.
While the multicoupler taught in U.S. Pat. No. 3,124,768 is suitable for many applications, it nevertheless poses difficulties which have not heretofore been easily solved. A first difficulty of the prior art device is that the arrangement of cavity filters and quarter wavelength transmission lines required to act as transformers, require friction couplings to electrically join the cavity filters, the transmission lines, and the other components into a unified system. It is well known that friction couplings create intermodulation interference problems: the greater the number of friction couplings, the greater the intermodulation interference. Additionally, it is well recognized that transmission lines introduce insertion losses which create desensitization problems in the multicoupler. Since the prior art device taught in U.S. Pat. No. 3,124,768 requires a multiplicity of quarter wavelength transmission lines and a multiplicity of friction connectors, both intermodulation interference and insertion loss problems are present.
Thus, it is evident that an improved multicoupler with reduced numbers of required transmission lines and friction couplings is needed to reduce to a minimum the intermodulation interference and insertion loss problems of the prior art devices. Obviously, a multicoupler having smaller numbers of these components will also have the advantage of being significantly less expensive.
Typical prior known multicouplers utilize standard cavity pass band and notch filters as the resonating components in their networks. A standard notch cavity filter includes an electrically resonant cavity with a moveable co-axial electrically conducting center probe for tuning the resonant frequency and a coupling probe connected at one end to the transmission line and grounded at its opposite end on the interior of the cavity. In a multicoupler, the standard notch filter acts as a short circuit in the transmission line spaced off a quarter wave from the junction at which the high impedance is desired. Varying the position, length, profile, etc., of the coupling probe permits the inductive coupling between the cavity and the transmission line to be increased or decreased. Such variation of the inductive coupling increases or decreases the loading of the cavity and hence decreases or increases the impedance or depth of the notch of the filter.
While such adjustability is desireable, standard prior art notch cavity filters have the deficiency that variation of the notch depth by adjustment of the grounded electrical probe causes the resonant frequency of the cavity to shift. When the notch of the notch filter shifts in this manner, it detrimentally effects the performance of the multicoupler. Accordingly, if one wishes to vary the impedance of the reject band of the notch filter of prior art multicouplers, not only would the inductive coupling between the grounded coupling probe and the cavity have to be adjusted, but also the resonant frequency of the cavity itself would have to be adjusted so as to shift it back to the frequency of the respective signaling device.
Accordingly, in prior art multicouplers, adjustment of the multicoupler to obtain additional isolation or reduced isolation of a particular frequency involves a complicated readjustment of not only the inductive coupling of the cavity but also of the cavity resonant frequency. Conversely, modification of a multicoupler to accommodate a change in frequency of the carrier signal of the signaling device requires a dual adjustment of tuning the resonant frequency of the cavity and then varying the inductive coupling of the grounded probe into the cavity so as to compensate for the electrical effect caused on the depth of the notch by the variation of the resonant frequency of the cavity.
It is evident therefore that a notch filter having notch depth tuning characteristics and frequency tuning characteristics independent of one another is desireable and would be especially useful in the context of a multicoupler. With such a notch filter, the multicoupler could be adjusted and tuned in a variety of ways without involving a complicated interdependent fine tuning operation.